The present invention relates generally to burst communications systems and, in particular, to burst communications systems having guard times and guard bands.
Burst communications systems typically employ multiple transmitters that send xe2x80x9cburstsxe2x80x9d of transmissions to a receiver. Bursts from the different transmitters are coordinated in time and frequency such that each of the transmitters can communicate with the receiver without interfering with each other.
Receivers for use in burst communications systems can be complex, in part because receivers re-acquire burst transmissions each time they are transmitted. The re-acquisition can be more complex when burst communications systems use signals having low probability of detection (LPD) and low probability of interception (LPI), such as direct sequence spread spectrum (DSSS) signals.
Additional complications arise when the transmitter and receiver are moving relative to each other, as is the case in satellite systems. When a burst communications transmitter is on or near the earth, and the intended receiver is in a satellite, the spatial locations and the relative velocities of the transmitter and receiver change over time. The change in spatial location causes the propagation path length and the signal propagation time to change, and the change in relative velocities causes a Doppler frequency to change the frequency of the burst signal when it is received at the intended receiver.
Receivers in burst communications systems typically have xe2x80x9cguard timesxe2x80x9d surrounding the expected arrival time of a burst communication. As the propagation path length changes, the burst communications signal occupies portions of the guard time, and the receiver typically includes additional complexity that allows the acquisition of the signal during the guard times. Moreover, these guard times and guard bands can also reduce the effective communications bandwidth. Receivers also typically have xe2x80x9cguard bandsxe2x80x9d in frequency that surround the expected arrival frequency of the burst communication. As the relative velocities between the transmitter and receiver change, portions of the guard bands can be occupied by the received burst communications signal, due to the Doppler frequency shift.
Burst communications receivers include additional complexity when they are able to acquire signals in guard bands and during guard times. It would be desirable for guard times and guard bands to be reduced in burst communications systems.
Accordingly, a significant need exists for a method and apparatus for reducing guard times and guard bands in burst communications systems.